A common technique to increase the transmission capacity of today optical communication systems is wavelength division multiplexing (WDM), wherein a plurality of optical channels, each having a respective wavelength, are multiplexed together in a single optical medium, such as for example an optical fiber. The WDM channels may be closely spaced (dense WDM or DWDM, having typical channel separation of 100 GHz-about 0.8 nm- or 50 GHz-about 0.4 nm or less) or coarsely spaced (CWDM, having typical channel separation greater than 5 nm) or a combination thereof.
Optical networking is expected to be widely used in perspective optical communication field. The term ‘optical network’ is commonly referred to an optical system including a plurality of point-to-point or point-to-multipoint (e.g., ring) optical systems optically interconnected through nodes. In all-optical transparent networks few or no conversions of the optical signal into electrical signal, and then again in optical signal, occur along the whole path from a departure location to a destination location. This is accomplished by placing at the nodes of the optical networks electro-optical or optical devices which are apt to process the optical signal in the optical domain, with limited or no need for electrical conversion. Examples of such devices are OADMs, branching units, optical routers, optical switches, optical regenerators (re-shapers and/or re-timers) and the like. Accordingly, the term ‘optical processing’ for the purpose of the present description is used to indicate any optical transformation given to an optical radiation, such as optical filtering, extracting a channel or a power portion of said channel from a set of WDM channels (‘dropping’), inserting a channel or a power portion of said channel into a WDM signal (‘adding’), routing or switching a channel or its power portion on a dynamically selectable optical route, optical signal reshaping, retiming or a combination thereof. In addition, optical systems, and at a greater extent optical networks, make use of optical amplifiers in order to compensate the power losses due to fiber attenuation or to insertion losses of the optical devices along the path, avoiding the use of any conversion of the optical signal into the electrical domain even for long traveling distances and/or many optical devices along the path. In case the WDM wavelengths are closely spaced (e.g. within about 30 nm bandwidth around 1550 nm), all channels are typically optically amplified together.
In optical systems, and at a greater extent in optical networks, a problem exists of processing one or more optical channels at the nodes minimizing the loss and/or the distortion of the processed optical channel(s), as well those of the optical channels transmitted through the node ideally without being processed. Advantageously, the optical processing node should be able to simultaneously process more than one channel, ideally nearly all the channels. In addition, it is highly desirable that the optical processing node is tunable or reconfigurable, i.e., it can change dynamically the subset of channels on which it operates. It is highly desired that while the processing node “moves” from the initial channels subset (A) to the destination channels subset (B), the channels different from A and B (hereinafter referred to as ‘thru channels’) remain unaffected by the tuning operation. In this case the component is defined as ‘hitless’. In particular, the thru channels placed between one of the initially processed channels and the respective final channel after tuning should not be subject to an additional impairment penalty, called ‘hit’, by the tuning operation. The hit may include a loss penalty and/or an optical distortion such as phase distortion or chromatic dispersion.
For example, optical communication networks need provisions for partially altering the traffic at each node by adding and/or dropping one or several channels out of the total number. Typically, an OADM node removes from a WDM signal a subset of the transmitted channels (each corresponding to one wavelength), and adds the same subset with a new information content, said subset being dynamically selectable.
There are several additional concerns. The tunable optical processing node should not act as a narrow band filter for the unprocessed channels, since concatenation of such nodes would excessively narrow the channel pass bands. The tunable optical processing node should also have low transmission loss and low cost, since these important factors ultimately determine which technology is selected.
U.S. Pat. No. 6,035,080 discloses a reconfigurable add-drop optical multiplexer (R-ADM) including at least one reconfigurable add-drop unit that can add-drop one channel out of a large set by switching the light path through one of a set of fixed add-drop filters (ADF). The ADFs comprise a pair of Bragg reflecting waveguides coupled together by a pair of spaced apart 3 dB couplers. The selection among add-drop filters is done by sliding an integrated optic chip with the set of ADFs between input and output waveguides. The sliding chip switch by itself cannot be used to reconfigure the ADM because it will disrupt the signal traffic as the chip is slid from one ADF to another. Instead, before making this change, it is desirable to switch from the add-drop filter path to a bypass path that is off the sliding chip without losing any bit. Reconfiguration is done by switching from the add-drop filter path to a bypass path, changing to a different add-drop filter and then switching back. In cited patent, a bypass switch is described comprising a pair of waveguide paths connecting a pair of identical thermo-optic switches, each one comprising a pair of 3 dB couplers connected by a pair of waveguide arms. Arms in first path include heating element to control the arm optical path-length (and thus phase). Arms in second path can provide a phase shift of π as compared with unheated arms in first path. In operation, the thermo-optic switches determine which path input light will take. During switching from one path to another, the lightwave separates into two paths, and these contributions interfere when the two paths come together again. Thus the transmission depends on the phase difference of the two paths. According to the cited patent, the phase delay of the bypass path is adjusted to maintain nearly maximum transmission during switching via a phase shifter placed in the bypass path. A number of such low loss reconfigurable ADM units can be put in series to independently add and drop that number of channels out of a larger set.
The Applicant has noted that the ADM described in the cited patent is not really hitless, in that there is a loss of about 1 dB in the channels neighboring the add-dropped channel during switching between the first and second paths (FIG. 11 of cited patent). An optical field with frequency near the stop band of the Bragg reflectors will undergo a phase change significantly different from a field with frequency far from the stop band (FIG. 10 of cited patent).
In patent application US 2005/0031260 it is described, with reference to FIG. 4, a variable optical delay line comprising a continuous delay element having two variable delay arms. The first arm comprises a 0 to T continuous delay and the second arm comprises both a 0 to T continuous delay and a fixed delay T.
The Applicant has found that there is a need for an optical communication system having optical processing functionality which is tunable and hitless. In particular, the hit loss during tuning should be less than or equal to 1 dB. Moreover, the optical processing node should leave unaltered the thru channels during tuning. In particular, it is desired that the optical processing node introduces no or low chromatic dispersion to the thru channels. In addition, the optical processing node should preferably leave unaltered the unprocessed channels during processing operation and should be low-loss, low-cost, fast tunable and/or broadband.